When region-overflow is set to auto, content overflowing in the last region should obey the value of the overflow property. Visible and hidden are properly processed, but for overflow: scroll, no scrollbar is drawn and the content cannot be scrolled using the mouse scroll wheel.
Created attachment 145106 [details] Ref test highlighting the problem
Also, when explicitly setting the overflow property of the last region to overflow: scroll; even though space is reserved for scrollbars, they can't be used (they're disabled, no "thumb" is drawn). Another thing to note here: DOM JavaScript properties related to scrolling return incorrect values or just don't work. This include things like element.scrollHeight, or setting element.scrollTop.
Another note: horizontal overflow (with lrtb writing mode) doesn't work either, when specifying overflow: auto or overflow-x: scroll. (Note that, in this case, the overflow property should apply to any region in the region chain, not just the last one)
Still repros on today's nightly (r151543)
Fixed in latest nightly, most likely due to the latest fixes in the regions && scrolling area.